Presently known compound check valve devices are usually provided with a pair of oppositely disposed inlet ports, to which pressurized fluid is supplied, and an outlet or delivery port to which the pressuized fluid is delivered, and a valve member operable responsively to higher pressure at one of the inlet ports for connecting the higher pressure port to the outlet port. The length of the valve member is such that when it occupies a neutral position (because of equal pressures at both inlet ports) both said inlet ports are in communication with the outlet port. However, if pressure increases at one of the inlet ports so that the valve member begins to shift, if such increase occurs at a relatively slow rate, the higher pressure from said one inlet port may feed back to the lower pressure inlet port.
The above-discussed problem was proposed to be solved by suggesting a compound check valve as shown in FIG. 1 and labeled PRIOR ART.
The compound check valve shown in FIG. 1 comprises a valve carrier 1 reciprocably disposed in a bore 2 formed in a housing 3 between a pair of oppositely disposed inlet ports 4 and 5, to which pressurized fluid is supplied for delivery to an outlet or delivery port 6. The valve carrier 1 is disposed in bore 2 in such a way that it can slide freely in the direction of either of the aforesaid inlet ports 4 and 5. When valve carrier 1 is in a neutral position, communications between inlet ports 4 and 5 and delivery port 6 are shut off. Valves 7 and 8 facing the aforesaid inlet ports 4 and 5, are mounted in annular spaces 9 and 10, respectively, formed coaxially in valve carrier 1. A pair of springs 13 and 14 normally serve to bias valves 7 and 8 against respective stop rings 11 and 12 fixed coaxially in valve carrier 1 when no fluid pressure is present at the inlet ports 4 and 5.
Annular valve seats 15 and 16 with which valves 7 and 8 are adapted for making sealing contact, are fixed in housing 3 in surrounding relation with inlet ports 4 and 5, respectively. When fluid pressure is supplied concurrently to both inlet ports 4 and 5 and one of the pressures is higher than the other, at inlet 4, for example, the higher pressure causes movement of valve carrier 1 toward the lower pressure inlet port 5 to cause valve 8 to be seated on valve seat 16. The higher pressure at inlet port 4, therefore, is communicated with delivery port 6 via passageways 17 opening from bore 2 into said delivery port. In this way, the conventional compound check valve, as just described, operates to detect the higher fluid pressure from one of the inlet ports, in this case inlet port 4, and to supply said higher pressure to delivery port 6. If the pressure at inlet port 5 is higher, the converse operation occurs. If the fluid pressures at both inlet ports 4 and 5 are substantially the same, valve carrier 1 normally stops at its neutral position to cut off communications between both inlet ports 4 and 5 and the delivery port 6. Moreover, even though valve carrier 1 may not stop at the neutral position, it may vacillate in bore 2 to establish a more or less cut-off condition.
The check valve device immediately above described is not necessarily desirable in that it either cuts off delivery of fluid pressure to delivery port 6 completely, or it communicates only one of the inlet ports 4 and 5 at a time to said delivery port.